DESCRIPTION: Despite many shared goals, public health and public safety systems often do not collaborate and therefore may end up working at cross purposes. Their divergent responses to sex work are particularly striking, with public health focusing on health promotion and risk reduction in contrast to public safety's focus on criminalization of both sex work and the substance use that so often accompanies it. The need to better understand and align these approaches is evident when one considers female sex workers' (FSWs) HIV risk. To fill this gap, study aims are to: 1) to explore police culture, attitudes of, and experiences with FSWs among Baltimore city police leadership (N=15) and street-level police (N=30) working in high crime areas in Baltimore City; 2) to examine the association between the FSWs' HIV risk behaviors (e.g., unprotected sex, unsafe injection practices use, crack use) with police encounters the frequency and type (i.e., verbal and sexual harassment, sexual exploitation, condom and syringe confiscation) of police encounters, among a cohort of FSWs (N=250) over a 12-month period of time; and 3) to examine the association between HIV/STIs incidence and prevalence with the frequency and type of police encounters among a cohort of FSWs (N=250) over a 12-month period of time. In meeting Aim 1, we will conduct organizational ethnography, a multi-method approach, which will allow for an in-depth examination of police culture, police policies, as well as attitudes and experience of leadership and patrol police in their own words. Through Aim 2, we will enroll and follow a cohort of FSWs (N=250) through targeted sampling, at street-based venues that will be identified through secondary data analysis, street key informant interviews, and ethnographic mapping. Through these data, we will examine associations between risky drug and sexual behaviors (i.e., unprotected sex, injection drug use) with police encounters over time. Data will be collected at baseline, 3-, 6-, 9-, and 12-months. The proposal is innovative in responding to an emerging, global evidence base illustrating the centrality of police in FSWs' HIV/STI risk behaviors and infection.1,2,9,19,20 The study blends organizational ethnography, prospective quantitative research, and biological outcomes and represents the first US-based effort to better understand how and why police function as a structural HIV/STI driver among FSWs, and in turn lay the foundation for future interventions. The study will be guided by a multi-sectorial community advisory board (CAB) comprised of police, legal, public health, FSW, and community stakeholders. The CAB and support of the Police Commissioner will be central to the development of sustainable and realistic interventions stemming from this work.